Search for the Higgs boson in the 4 leptons channel at ATLAS. · Search for the Higgs boson in the 4 leptons channel at ATLAS. Nicolas Morange CEA-IRFU (Saclay) ... b-quarks mass

Search for the Higgs boson in the4 leptons channel at ATLAS.

Nicolas Morange

CEA-IRFU (Saclay)

SLAC seminar

September 24, 2012

Starting with the conclusion

You all know the conclusion...

Discovery of a new particle in the search for the SM Higgs boson

4 leptons channel one of the major players in game

Published Phys.Lett., B716 (2012) 1-29

Nicolas Morange (IRFU) SLAC seminar 24/09/2012 2 / 44

Starting with the conclusion

You all know the conclusion...

... which has been achieved after (only) 2.5 yearsof data-taking !


Thanks to... the LHC

Ecm:

900 GeV (2009),7 TeV (2010,2011),8 TeV (2012),13–14 TeV (2015-)

Instantaneous luminosity:

∼ 1032 cm−2s−1 (2010),∼ 1033 cm−2s−1 (2011, 2012),1034 cm−2s−1 (nominal)

Spacing between proton bunches:

50 ns (2010-2012)25 ns (nominal)

2012: 15 fb−1 and ongoing

Integrated luminosities:

2010: 45 pb−1

Day in 2011

28/02 30/04 30/06 30/08 31/10

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tal In

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LHC Delivered

ATLAS Recorded

1Total Delivered: 5.61 fb1Total Recorded: 5.25 fb

2011: 5.25 fb−1


Thanks to... the ATLAS detectorMuon Spectrometer: (|η|< 2.7)Air toroid with drift chambers,Provides μ trigger and momentum measurement,Resolution < 10% up to p∼ 1 TeV.

Inner Detector: (|η|< 2.5, B=2T)Si Pixels, SCT, TRTPrecision tracking,Vertex reconstruction,e/π separationσ/pT ∼ 3.810−4pT⊕0.015

Hadronic Calorimeter:Scint/Fe tiles in barrel (|η|< 1.7)W/Cu-LAr in endcaps (|η|< 4.9)Provides jet trigger and energy measure-ment,σ/E∼ 50%/

pE⊕3%

Hermetic coverage for MET

EM Calorimeter: (|η|< 3.2)Pb-LAr, accordion structureProvides trigger on e/γ,Identification and measurementσ/E∼ 10%/

pE⊕0.7%

Trigger System:3 levelsL1: calo and muons, 75 kHzdedicated electronicsL2: all detectors, 4 kHzfast reconstructionEF: all detectors, 300 Hzfull reconstruction


Discovering a new particle: a long journey

2.5 years of data-taking to:

Commission and understand finelythe detector

Understand and improvereconstruction performance

Measure basic SM processes("re-discover" the SM)

Improve and perform the search forthe Higgs

ATLAS simulation 14 TeV, 2008< 5σ at 125 GeV with 10 fb−1.

All of these steps absolutely crucial to obtain thecurrent results !


Going through the steps

1 Commissionning the ATLAS detector: the L1Calo trigger system at highenergies

2 The Higgs boson at the LHC

3 Measuring basic processes: b-jets associated with Z bosons

4 Improving and performing searches: the H→ 4ℓ channel







The electromagnetic calorimeter

Geometry

sampling Pb/LAr calorimeter, accordionstructure

contains EM showers

Barrel |η|< 1.47, endcaps1.37< |η|< 3.2

3 layers in depth, plus a presampler

fine transverse segmentation

⇒ ∼170 000 cells

Calorimeter Readout

Drift time 400 ns

⇒ bipolar shaping, rise time 45 ns

Optimal filtering

⇒ energy, timing, signal quality

ATLAS


The L1 calorimeter trigger system

Search for deposits compatible with physicsobjects

Computation of transverse energies,

Bunch-crossing identification (BCID).

Trigger towers

Lowered granularity: mostly∆η×∆ϕ= 0.1×0.1,

Obtained by analog sums (dedicatedelectronics, Saclay involvement),

Signals digitized at 40 MHz on 10 bits, withsteps of 250 MeV,

Energy computation: Finite Input Responsefilter (FIR ∼ optimal filtering) with 5 samples,

BCID: look for maximum of FIR output.

Trigger logic

Find Regions of Interest (local maxima),

If event accepted, transmit to L2.

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The trigger system at high energies

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At very high energies:

Saturation: digital range 256 GeV. Large pulses may also be distorted

Digital saturation ⇒ L1 trigger fired. But on which bunch-crossing ?

BCID for saturated pulses:Use a threshold algorithm

1st saturated

sample

Sat. algo decision

FIR decision

L1 decision

High Threshold

Low Threshold

comparison

comparison

3953339033 34302FIR weighted sums:


Commissioning the Trigger for large pulses

Problem: initial calibration of threshold algorithm valid only for very large pulses (eg500 GeV), leaving "hole" in trigger range in 250–500 GeV

Solution: changing the trigger logic

Leave FIR algorithm on beyond 250 GeV: actually works at least up to 700 GeV

Trigger BCID: the earlier of the two outcomes

⇒ Then must prove the whole range is covered with high efficiency

time [ns]

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Difficulties with thresholds algorithmcalibration

Sensitivity to electronics noise,

Sensitivity to digitization timings,

Shape differences between calibration andphysics signals.


Results and consequences

Validation

Uses both calibration and physics data

Makes use of the linearity of the system

Must be done for each Trigger Tower and each data-taking period

Results

BCID validated up to 3.5 TeV for most trigger towers,

Computation of uncertainties per period, for barrel and endcaps,

⇒ Application: uncertainty on W′ (1 % ) and Z′ (1.8 %) searches

Changes in trigger configuration (thresholds) for 2011 data-taking:

Better behaviour at the highest energies,⇒ Uncertainties become almost negligible.

Uncertainty on trigger efficiency fora deposit of E= 3.5 TeV (%):

5 problematic towers,

some with uncertainties∼ 5%

η and ϕ structures due to detectoreffects.

ATL-DAQ-INT-2011-001 (ATLASmembers) or CERN-THESIS-2012-087 η

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The Higgs boson at the LHC

Production and decay

3 main production modes,cross-sections known to NLO or NNLO,

Many decay modes, branching ratiosknown to NLO,

Importance of different channelsdepends on hypothesised mass:ZZ, WW, γγ, V+H(bb̄), ττ,. . .

H→ ZZ: different final states, includingZZ→ 4ℓ


The Higgs to 4 leptons channel

H→ ZZ→ 4ℓ

Decays Z→ ee and μμ: 3 channels, 4e,2e2μ, 4μ

⇒ low branching ratio:σ×BR∼ 1−10 fb

Excellent resolution on MH: ∼ 2 GeV

Very clean signature, low backgrounds

⇒ Important channel on 120–600 GeV

Backgrounds

Irreducible background: ZZ

Backgrounds with leptons fromheavy-flavor decays: tt̄, Zbb̄

Backgrounds with fake leptons:Z+light jets

Zbb̄ background

Uncertainties on QCD computations of itsproduction (see W+b at Tevatron):

Multi-scales process: Q2, MZ , Mb

b-quarks mass effects

Uncertainties on processes withb-quarks in the initial state







Zbb̄: b-quarks in the initial state

Origin of the issue: incompatibility between quark masses and PDF

2 different schemes

Variable number of flavors (SHERPA): use a b PDFabove threshold

Good behaviour at large Q2

Difficulties around threshold

Fixed number of flavors (ALPGEN): b created bygluon-splitting only

Needs special 4 flavors PDF.Kinematics correct around threhsoldLacks logarithmic resummations at large Q2

Xp

p

Y

X(b̄)

b

Xp

p

X

Y

b̄b

Merging the schemes

At NLO, possibility to interpolate between theschemes

Good behaviour in both limitsSmooth transitionSeveral possible prescriptions: S-ACOT(MCFM)


The Z+b analysis

Preparing the H→ 4ℓ analysis: control Zbb̄

Early data (2010) : 36pb−1 ⇒ allows measurement ofZ+b−jet

Test QCD prediction for this type of events

Background to other Higgs and SUSY searchesOn longer term, measurement of b-jets energyscale, and of b PDF.

b

b

Z

g

q Z

b

bq

In a nutshell:

Inclusive measurement on b jets

Cross-section of b-jets in Z events:

σb =N(b-jets)

LAN(b-jets) measured after b-tagging and statistical fit.

Ratio σb/σZ (number of b jets per Z event) alsomeasured


Selection of events

Standard Z selection:

Single lepton triggers (e, μ)

Good quality leptons, pT > 20 GeV, |η|< 2.5

Pair of leptons, 76<mℓℓ < 106 GeV

Adding (b-)jets:

Anti-kT , parameter 0.4, pT > 25GeV, |y|< 2.1

Separated from leptons by ∆R> 0.5

b-tagging: reconstruct a secondary vertex

Cut on the significance of the distanceto primary vertexCalibrated for 50 % efficiency

Simulations

Z+jets (signal andbackgrounds)SHERPA and ALPGEN

tt̄ MCNLO

Dibosons ALPGEN

Single top MCNLO

Simulations corrected to match dataas best as possible

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Invariant mass Z→ ee

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b-tagged jets spectrum(Z→ ee)


Selection of events

Standard Z selection:

Single lepton triggers (e, μ)

Good quality leptons, pT > 20 GeV, |η|< 2.5

Pair of leptons, 76<mℓℓ < 106 GeV

Adding (b-)jets:

Anti-kT , parameter 0.4, pT > 25GeV, |y|< 2.1

Separated from leptons by ∆R> 0.5

b-tagging: reconstruct a secondary vertex

Cut on the significance of the distanceto primary vertexCalibrated for 50 % efficiency

Simulations

Z+jets (signal andbackgrounds)SHERPA and ALPGEN

tt̄ MCNLO

Dibosons ALPGEN

Single top MCNLO

Simulations corrected to match dataas best as possible

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tt) + jetsττ→Z(

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Invariant mass Z→ μμ

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Number of jets (Z→ μμ)


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b-tagged jets spectrum(Z→ μμ)


Results of the event selection

In data:

Selection Electron Muon≥ 1 b-tag 64 67= 1 b-tag 62 63= 2 b-tag 1 4= 3 b-tag 1 0

Data-MC agreement:

Reasonable

For both generators used

In electron and muon channels

For both yields and spectra.

Backgrounds:

Selection purity: ∼ 50%

Z+c and Z+ l dominant (taggingefficiency)

Important contribution tt̄

Dibosons, single top sub-leading

Contribution of multijets (1 event):data-driven

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DibosonSingle top

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ATLAS

Invariant mass Z→ ee, b-tagged jet


Extraction of the number of b-jets

Issue:

Purity not sufficient to measure cross-section directly

⇒ Need of a statistical extraction

Main backgrounds made of Z+non-b-jets

⇒ Chose invariant mass of tracks associated to the secondary vertex asdiscriminant variable

Method

Muon and electron channels added,

Templates for signal and backgroundstaken from MC,

Sub-leading backgrounds (mainly realb-jets) normalized to their estimatedcontributions,

Normalizations of signal and dominantbackgrounds are the result of the fit.

Method validated with pseudo-experiments.

SV0 mass [GeV]0 1 2 3 4 5 6 7 8 9 10

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1 L dt = 36 pb∫

Fit results

Nb = 63.6+14.7−13.2 Nc = 59.9+13.4

−14.0Nl = 0.0+5.1

−0.0 N(other) = 14.5 (fixed)


Systematic uncertainties

Uncertainties affect result inseveral ways

Distorsion of templates

Normalization ofsub-leading backgrounds

Change in acceptance(efficiencies)

The final result takes correlationand asymmetric uncertaintiesinto account.

Main sources:

Tagging efficiency and modeling⇒ will significantly decrease.

Source Fit (%) Acceptance (%)

Electron and MuonTagging efficiency 1.7 9.1Template modeling 3.5 -Model dependence 2.7 10.0Jet energy scale 0.7 4.0σtt̄ 2.0 -MPI negl. 1.0

Electron onlyMC statistics negl. 1.3Multijets background 1.6 -Electron efficiency negl. 5.0Total Electron 5.6 15.0

Muon onlyMC statistics negl. 1.3Multijets background 0.7 -Muon efficiency negl. 2.0Total Muon 5.4 14.3

Total uncertainty +21% -16%


Results

Measurement of the fiducial cross-section σb

Theory (MCFM): partonic NLO + hadronization corrections

Theory uncertainties: PDF, renormalization scale, αs

Experiment 3.55+0.82−0.74(stat)+0.73

−0.55(syst)±0.12(lumi) pb

MCFM 3.88±0.58pb

Published in PhysicsLetters B706:295-313,2012.

Measurement of the ratio σb/σZ:

Mean number of b jets per Z event

Interest: Direct comparison with LO generators

Experiment (7.6+1.8−1.6(stat)+1.5

−1.2(syst))×10−3

MCFM (8.8±1.1)×10−3

ALPGEN (6.20±0.2(stat only))×10−3

SHERPA (9.5±0.1(stat only))×10−3

MCFM: good agreement withmeasurement

LO generators: both at 1 sigma,large Alpgen/Sherpa difference

⇒ Advocates the use of k-factorsspecific to Z+b.

⇒ Actually used for backgroundevaluation in H→ 4ℓ december2011 search







Search for H→ 4ℓ with 11fb−1 of data

Data used:Good quality data recorded by ATLAS in 2011and 2012

4.9 fb−1 for 4e in 2011,

4.8 fb−1 for 4μ and 2e2μ in 2011,

5.8 fb−1 in 2012.

High pile-up environment: up to <μ>= 30.

Principle of the search:

Select events with 2 pairs ofleptons

Reconstruct a candidate byapplying cuts on invariant masses

Further reduce backgrounds withcuts on isolations and impactparameters

Look for a peak in the 4 leptonsinvariant mass distribution, over acountinuous background made ofZZ and reducible backgrounds.

Sensitivity depends on:

Signal reconstruction efficiency

⇒ Lepton reconstrucitonefficiency

Peak width

⇒ Lepton resolution

Backgrounds rejection and control

Analysis dominated by leptonperformance


Improving the electron reconstruction

Low cross-sections involved in H4l ⇒ every increase of efficiency counts.

An improved electron reconstruction (ATLAS-CONF-2012-047)

Until 2011, track reconstruction does not take Bremsstrahlung into account⇒ loss of tracking efficiency at low pT⇒ poor estimation of track parameters.

Improved reconstruction for ATLAS, effort led by H→ 4ℓ analysis:⇒ Refit of existing electron tracks (2011-)⇒ Plus account for Bremsstrahlung possibility in track finding (2012)

Improvements of track parameters in transverse plane (d0, ϕ, q/p)

Does not change the energy measurement (calorimeter only)

Consequence on the analysis

+10 % efficiency on the signal in 4e and 2μ2e after impact parameter cuts

further gains from new tracking

What about Zbb̄ and tt̄ backgrounds ?

Non-0 impact parameters ⇒ possible bias ?

Efficiency of impact parameter cuts ?


Validation for leptons from heavy-flavor decays

Validation on simulations

Study electrons from decays of b hadrons using Z+b (ALPGEN)Electrons with pT > 7 GeV, |η|< 2.5.

Unchanged d0distribution

Significantlyimproved d0resolution

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Validation for leptons from heavy-flavor decays

Validation with data

Select a sample enriched with electrons from HF decays

Z+b and tt̄ (dilepton) selectionsSelect electrons near (∆R< 0.5) tagged jets490 electrons selected ; purity: ∼ 60%.

Study electron properties

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Results

data-MC agreement as good as with the standard reconstruction.

Improvement of the observed resolution.


December 2011 results in a nutshellPhys.Lett. B710 (2012) 383-402:

H→ 4ℓ: Observed 95 % CLs exclusion curve close to the expected one: analysis wellunderstood

Exclusion of SM Higgs: 134–156 GeV, 182–233 GeV, 256–265 GeV and268–415 GeVExclusion expected: 136–157 GeV and 184–400 GeV

H→ 4ℓ: Deviations observed around 125 GeV, 244 GeV and 500 GeV (∼ 2.1σ)

ATLAS combination: non excluded 122.5 – 129 GeV and > 539 GeVATLAS combination: excess of 2.6σ at 126 GeV

⇒ If the SM Higgs exists, it is there !

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Exclusion limits, low mass [GeV]Hm

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December 2011 results in a nutshellPhys.Lett. B710 (2012) 383-402:

H→ 4ℓ: Observed 95 % CLs exclusion curve close to the expected one: analysis wellunderstood

Exclusion of SM Higgs: 134–156 GeV, 182–233 GeV, 256–265 GeV and268–415 GeVExclusion expected: 136–157 GeV and 184–400 GeV

H→ 4ℓ: Deviations observed around 125 GeV, 244 GeV and 500 GeV (∼ 2.1σ)ATLAS combination: non excluded 122.5 – 129 GeV and > 539 GeVATLAS combination: excess of 2.6σ at 126 GeV

⇒ If the SM Higgs exists, it is there !

Combined exclusion limitsCombined p0 values


Improving the H→ 4ℓ analysis

December 2011 analysis:

Very good results

Still not fully optimized for very low mass searches

⇒ Optimizations performed with simulations and 2011 data in control regions

HZ

Z

ℓ

ℓ̄

ℓ̄

ℓ

MH < 2MZ

⇒ Z off-shellAt very low masses(® 130 GeV), higher

probability for both Zoff-shell

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Adapting to the kinematics of low-mass Higgs

Some cuts need relaxing to gain acceptance

Needs good control of backgrounds

Reasonable for Zbb̄,Z+jets less well controlled, and significant in 4e and 2μ2e.


Optimizing light jets rejection

Light jets rejection

Quite low at low pT

Study one additional shower shape cutat pT < 10 GeV

Check simulations with a study of Z+eevents

Adding the cut

Optimization in η bins

Simulations (4e and 2μ2e channels):

Signal: -2 %Backgrounds: Z+jets -50 %, Zbb̄-40 %Significance: +15 %

Eratio

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Data 2011Zbb+jets

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ATLAS Internal1


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fraction b

elo

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Cut value

[0.0;0.1]∈η



Optimizing analysis cuts

Re-analyzing 2011 data

Optimization efforts carried separately for channels with subleading electrons ormuons (different backgrounds)

Here channels with subleading electrons shown

Optimizations done on simulations and control regions of 2011 data

Optimizations studied

Issue: low MC statistics for Z+jets⇒ Use control region to measure its variations

Invariant mass 1st pair of leptons: MZ−15 GeV → MZ−40 GeV

min cut [GeV]12M

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July 2012 analysis: Event selectionNumerous improvements wrt December 2011 analysis

Trigger:

Single or di-lepton triggers, thresholds evolving with luminosity

Lepton selection:

Electrons: pT > 7 GeV, |η|< 2.5, quality optimized for high efficiency

Muons: pT > 6 GeV, |η|< 2.7, maximizing the acceptance:

include spectrometer-only muons at |η|> 2.5include calorimeter-tagged muons at |η|< 0.1

Reconstruction of a candidate:

2 pairs of leptons, same flavor, opposite signs,

pT cuts at 20, 15 and 10 GeV for the 3 highest-pT leptons,

Angular separation ∆R> 0.1 for e-e and μ-μ, 0.2 for e-μ

Pair closer to the Z pole: 50<m12 < 106 GeV,

Second pair: mmin <m34 < 115 GeV.


Additional cutsGoal: suppress reducible backgrounds.

Lepton isolation Target: Z+jets, multijets

Relative isolations of tracks and clusters in cones of size ∆R< 0.2Take pileup into accountTake other leptons of the candidate into accountTrack isolation: (

∑

pT)/pℓT< 0.15

Calo isolation: (∑

ET)/EℓT< 0.30 for muons, 0.15 for standalone muons, 0.2 (2012)

or 0.3 (2011) for electrons

Transverse impact parameters Target: Zbb̄, tt̄

|d0/σ(d0)|< 3.5 (muons), 6.5 (electrons)

Efficiencies

Checked with Z→ ℓℓ decays (isolated leptons), Z+ ℓ and heavy flavor dijet events(non isolated leptons)Good data/MC agreement : ratios compatibles with 1Good simulation of pileup dependenceSmall systematic uncertainties

Final discriminant

Z mass constraint applied to leading lepton pair (m4ℓ < 190 GeV) or both(m4ℓ > 190 GeV): improvement of ∼10 % on resolution.


Signal: SimulationsGenerators:

Powheg+Pythia for gg and VBF.

Pythia for WH and ZH

Normalized to best NNLO/NLOcomputations available

Uncertainties: PDF and αs envelope(8 %), QCD scales (8 %)

Reconstruction and selectionefficiencies at 130GeV:

4μ 2e2μ 4e8 TeV data 41 % 27 % 23 %7 TeV data 43 % 23 % 17 %

december 2011 27 % 18 % 14 %

Resolutions at 130GeV:

1.8 GeV (4μ), 2.0 GeV (2e2μ), 2.4 GeV(4e)

4e: tails and lower mean value(Bremsstrahlung),

Higgs natural width dominates from350 GeV : width∼ 29 GeV àmH = 400 GeV.


Controlling backgrounds: ZZ∗

Production of SM ZZ∗:Irreducible background

Generated with Powheg (qq̄diagrams) and gg2ZZ (ggdiagrams)

Cross-section normalized to MCFMprediction

Uncertainties from QCD scales(5 %) and PDF+αs (4 % for qq̄, 8 %for gg)

Shape uncertainty from varying thegg contribution

q

q̄

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Controlling backgrounds: ℓℓ+μμ channels

Reducible bkg dominated by tt̄ and Zbb̄

Control region pure in thosebackgrounds: no isolation cuts,revert impact parameter cuts

Fit m12 distribution using analyticfunctions (shapes taken from MC,normalizations free)

Extrapolate to signal region usingMC ; cross-check transfer factors indata

Cross-check for tt̄: eμ+μμ controlregion

Data 4μ 2e2μZ+jets 8 TeV 0.51±0.13±0.16 0.41±0.10±0.13Z+jets 7 TeV 0.25±0.10±0.08 0.20±0.08±0.06tt̄ 8 TeV 0.044±0.015±0.015 0.040±0.013±0.013tt̄ 7 TeV 0.022±0.010±0.011 0.020±0.009±0.011


Controlling backgrounds: ℓℓ+ee channels

Reducible bkg dominated by Z+jets

Control region: relax identificationof subleading electrons. ∼100events per year and per channel

Categorize electrons inelectron-like, conversion-like andfake-like using info from trackerand calorimeter (9 categories)

Extrapolate contamination in signalregion from these categories usingMC

Two other methods used forcross-checks

Data 4e 2μ2eBackground 8 TeV 3.9±0.7±0.8 4.9±0.8±0.7Background 7 TeV 3.1±0.6±0.5 2.6±0.4±0.4



Normalizations:

Effects of PDF, αs and QCD scales,

Additional uncertainty for high mass searches: 150%×m3H[TeV] for mH > 300 GeV,

Luminosity: 3.6 % (2012), 1.8 % (2011).

Reducible backgrounds:

ℓℓ+μμ channels: 50 %,

ℓℓ+ee channels: 25 %,Origin: Statistical uncertainties in control regions, uncertainties on the methodsthemselves and on the extrapolations

Reconstruction and selection efficiencies:

Muon efficiency: 0.16 % (4μ), 0.12 % (2e2μ),

Electron efficiency: 3.0 % (4e), 1.7 % (2e2μ) at 600 GeV. 8.0 % and 4.6 % at 110 GeV,

Lepton resolution: negligible,

Electron energy scale: uncertainty 0.7 % (4e) and 0.4 % (2e2μ) on mass scale.

Isolations and impact parameters: negligible



Normalizations:



Luminosity: 3.6 % (2012), 1.8 % (2011).












Normalizations:



Luminosity: 3.6 % (2012), 1.8 % (2011).











Results: Phys. Lett. B 716 (2012) 1-29

Events in window 120–130GeV

Signal ZZ(∗) Z+ jets, tt̄ Observed4μ 2.09±0.30 1.12±0.05 0.13±0.04 6

2e2μ/2μ2e 2.29± 0.33 0.80±0.05 1.27±0.19 54e 0.90±0.14 0.44±0.04 1.09±0.20 2

Reducible backgrounds still large in 4e/2μ2e channels

Data exceed background expectations around 125 GeV

Invariant mass distribution Masses of the reconstructed Z pairs


Event display

4μ candidate. m4ℓ = 123.5 GeV.Masses of the lepton pairs: 84 GeV and 45.7 GeV.


Investigation of the excess

Test statistic: maximum likelihood fit to data using signal and background histograms.

H→ 4ℓ Exclusion limits set with CLs formalism:

Expected 124–164 GeV and 176–500 GeVObserved 131–162 GeV and 170–460 GeV

One significant excess in mass range: p0 of 3.6σ at 125 GeV (2.7 expected)

⇒ Evidence for new particleWith look-elsewhere effect in 110–150: global p0 of 2.5σ

ATLAS combination: observation at 6.0σ of a new particle of mass 126.0±0.6 GeV

signal strength parameters compatible with 14ℓ and γγ masses compatible

H→ 4ℓ low-mass exclusion limits H→ 4ℓ high-mass exclusion limits










H→ 4ℓ p0 values H→ 4ℓ (μ,mH) contour










Combined p0 values Superposition of (μ,mH) contours


What next ?

Have we discovered the SM Higgs ?

Goal: Test compatibility of SM oralternative theories to themeasurements

What can we achieve ?

Measurement of mass

Determination of spin

Determination of parity

Measurement of the couplings

How ?

Go for precision measurements inthe "discovery" channels

Look for a signal in other modes:H→ bb̄, H→ ττ, tt̄H of primeimportance

Look for exclusive signatures: VBF,associated production

Lot of work ahead !


Conclusions

Discovery of a new Higgs-like particle : a major result

It took 20 years of preparation, and 2.5 years of data-taking to achieve it

Commissioning ATLAS: high energy pulses at L1

An important part of the detector to get ready at the start of data-takingDifficult validation, which led to changes in trigger logicDirect consequences for some exotic searches

Prepare for the H→ 4ℓ search: Z+b measurement with 2010 data

Measurement with 30 % uncertainty, in agreement with NLOGives confidence in evaluation of Zbb̄ background in the H→ 4ℓ channel:computation of k-factor applied on ALPGEN samples

Searching for the Higgs: the 4 leptons channel

Lepton performance of prime importanceDevelopement of a new electron reconstruction

After december 2011 results, series of optimizations for low-mass searchesImprovements on background rejection and optimized kinematical cuts

Search with 2011 and 2012 data: 3.6σ excess at 125.0 GeVCompatible with SM Higgs and the excesses in other channelsA new field now opens: precision tests of SM and alternative theories

All decay channels should be studied to obtain the best possible precisionSoon 20fb−1 available and improved analyses in the high resolution channels


Backup slides

Basics of validation of the settings

Goal

Determine energy ranges where BCID algorithms are valid,

Compute lower bounds on trigger efficiency at high energy.

Constraints

Should be done for each trigger tower,

Should be done for each data-taking period,

Cannot rely on calibration data only,

Very small statistics of high energy deposits inphysics data.

Solution

Use calibration data to detect hardware issues,

Use calibration data to understand behaviour ofsignals at high energies,

Make use of signal linearities to extrapolate lowenergy physics data up to high energies.

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Estimation of multijets background

Data-driven estimateAssume an exponential invariant mass spectrum for this background.

Relax electron identification: 1 medium 1 candidate, or 2 loose no medium(cross-check)

Fit m(Z): function for multijets, MC distributions for signal and other backgrounds

Fit standard selection, with expo. slope from previous fit.

Extract number of events under Z peak: 1.0±2.2

ee mass [GeV]60 70 80 90 100 110 120

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Fit medium+medium

Muons channel: Use non isolated muon pairs. Result: N = 0.0±0.9.


Cross-section computation

Measurement: mean cross-section perlepton flavor

σb =Nb

AeLe +AμLμ

Nb: fit result

A: from simulation. Take into account:

Tagging efficiency,Jets reconstruction,Lepton reconstructionSmall extrapolation of leptonsacceptance.

ALPGEN and SHERPA numberscompatible.Aμ = 0.286, Ae = 0.214

L: luminosity: 36 pb−1.

Fiducial volume

Truth-level selections

Z boson:

Two leptons pT > 20 GeV, |η|< 2.5

Invariant mass in MZ±15 GeV

Jets:

Reconstrcuted from all final stateparticles, Z leptons excepted.

pT > 25 GeV, |y|< 2.1

Separated from Z leptons

Presence of a B hadron withpT > 5 GeV within ∆R< 0.3


Documents

Search for the Higgs boson in the 4 leptons channel at ATLAS. · Search for the Higgs boson in the 4 leptons channel at ATLAS. Nicolas Morange CEA-IRFU (Saclay) ... b-quarks mass